Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literal tire in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's co-pending application Ser. No. 09/503,881 (Now U.S. Pat. No. 6,614,914) and U.S. Pat. No. 5,862,260, which are hereby incorporated by reference. Examples of other watermarking techniques are described in U.S. patent application Ser. No. 09/404,292, which is hereby incorporated by reference. Additional features of watermarks relating to authentication of media signals and fragile watermarks are described in U.S. patent application 60/198,138, Ser. No. 09/498,223 (Now U.S. Pat. No. 6,574,350), Ser. No. 09/433,104 (Now U.S. Pat. No. 6,636,615), and 60/232,163, which are hereby incorporated by reference.
The invention provides a method of authenticating a media signal and related software, systems and applications. The method transforms at least a portion of the media signal into a set of frequency coefficients in a frequency domain. For example, it applies a Fast Fourier Transform (FFT) or other frequency transform to blocks of a media signal, such as an image, audio or video signal. It adjusts a relationship between selected frequency coefficients to a reference value. This adjustment is selected so that an alteration to be detected, such as a re-sampling operation or digital to analog—analog to digital conversion, alters the relationship. To detect the alteration, a detector computes the relationship in a potentially corrupted version of the signal. It then compares the result with a threshold value to detect whether the alteration has occurred.
A further aspect of the invention is a method of authenticating a media signal. The method evaluates signal peaks at selected frequency coefficients of the media signal. In a prior embedding process, the media signal has been modified to include peaks at the selected frequencies, such as by the technique summarized in the previous paragraph. The method determines, based on degradation of the signal peaks, whether the media signal has been altered. The frequency location of the peaks may vary from one application to the next. To detect, scanning and printing of watermarked images for example, the peaks are located at higher frequencies.
Another aspect of the invention is a watermark decoder, which includes a detector and analyzer for determining alteration of a watermarked media signal. The detector correlates a calibration signal with a media signal suspected of carrying a watermark to determine orientation parameters describing orientation of the media signal at embedding of the watermark. The calibration signal includes a set of peaks at selected frequency coefficients. The analyzer orients the media signal using the orientation parameters and evaluates whether the media signal has been altered by examining signal peaks at selected frequency coefficients in the media signal.
Further features will become apparent with reference to the following detailed description and accompanying drawings. The following description details a method for detecting whether an image has been scanned, printed or photocopied after being processed by the method. It also describes alternative implementations and applications.